1. Field of the Disclosure
The invention relates generally to wireless communication systems, such as multi-user Wi-Fi® systems, and more particularly to methods for providing feedback on channel observations from a wireless receiver to a wireless transmitter.
2. Related Art
Performance of a wireless network, e.g., a wireless network complying with an IEEE 802.11 standard, depends on channel noise and interference present on the wireless medium. Both channel noise and interference can cause decoding errors in received packets, thereby increasing the packet error rate (PER) in a receiving wireless station.
Channel noise is relatively constant background noise that interferes with the reception of a transmitted signal at a receiving wireless station. In general, channel noise cannot be decoded to provide a meaningful signal. Every object emits some spectrum of radiation waves (heat/energy), referred to as thermal noise, which contributes to channel noise. Another possible source of channel noise includes energy emitted from microwave ovens. In general, channel noise cannot be avoided by a receiving wireless station.
In contrast, interference, as defined herein, is an intermittently transmitted signal, which is generally capable of being decoded by some receiver, but which temporarily interferes with the reception of a transmitted signal at an intended receiving wireless station. Examples of interference signals include signals transmitted by ‘hidden nodes’ in a Wi-Fi system, signals transmitted on adjacent channels in a Wi-Fi system, and signals transmitted by other technologies, such as Bluetooth, which operate in the same frequency band as a Wi-Fi system. In general, interference can be avoided in some manner, for example, by techniques such as the request-to-send/clear-to-send (RTS/CTS) mechanism of the IEEE 802.11 specification (which addresses hidden node interference), changing the communication channel in response to the detected interference (which addresses interference from signals transmitted on adjacent channels), or implementing co-existence mechanisms (which allow interfering signals to share the channel with the receiving wireless station).
The receiving wireless station typically does not determine a packet error rate of a received packet. Rather, for each received packet, the receiving wireless station transmits a block acknowledge (BA) frame to the transmitting wireless station, wherein the BA frame includes a bit map that indicates whether or not specific sub-frames of the packet were successfully received. The transmitting wireless station calculates the packet error rate in response to the bit map of the received BA frame. The transmitting wireless station then uses the calculated packet error rate in a rate adaptation algorithm to select a modulation & coding scheme (MCS) used to transmit future packets. If the packet error rate becomes high, the rate adaptation algorithm determines that the channel condition has deteriorated, and in response, the transmitting wireless station reduces the physical layer (PHY) data rate (with higher coding gain) to transmit future packets.
As described above, the transmitting wireless station is informed of packet errors, but is not informed whether these packet errors occurred as a result of channel noise or interference. As long as the packet error rate increased as a result of channel noise, reducing the PHY data rate with higher coding gain should result in a reduced packet error rate for subsequently transmitted packets. However, if the packet error rate increased as a result of interference, and the transmitting wireless station reduces the PHY data rate of the subsequently transmitted packets, the transmission time of these subsequently transmitted packets will increase, undesirably increasing the chance that these transmitted packets will encounter more interference, further increasing the packet error rate. It would therefore be desirable to have a method and apparatus for overcoming this problem. More specifically, it would be desirable to have a wireless system that allows a transmitting wireless station to adjust the MCS used to transmit packets in a manner that takes into account whether an increased packet error rate is a result of channel noise or interference.